In a clinical examination for medical diagnosis, biochemical analysis and immunological analysis are performed on protein, sugar, lipid, enzyme, hormone, inorganic ions, disease markers, and the like in a biological sample, such as blood and urine. In addition, a wide variety of analysis methods is used, including a method using DNA probes and a method of measuring DNA and the like by means of electrophoresis. Since a plurality of inspection items needs to be processed with high reliability and at high speed in a clinical examination, most of the items are processed using an autoanalyzer. As the autoanalyzer, there has been conventionally known, for example, a biochemical autoanalyzer in which a reaction liquid prepared by mixing a desired reagent into a sample, such as blood serum, and reacting the reagent with the sample is used as an object of analysis to conduct biochemical analysis by measuring the absorbance of the reaction liquid. This type of biochemical autoanalyzer is provided with a container for housing samples and reagents, a reaction cell into which a sample and a reagent are injected, a pipetting mechanism for automatically injecting a sample and a reagent into the reaction cell, an automatic agitating mechanism for mixing the sample and the reagent within the reaction cell, a mechanism for measuring the spectrum of a sample the reaction of which is in progress or completed, an automatic cleaning mechanism for suctioning and discharging a reaction liquid after the completion of spectrum measurement to clean the reaction cell, and the like (see, for example, Patent Literature 1).
In the field of autoanalyzers, reducing the amount of sample and reagent has become a technical issue. That is, an amount of sample that can be allocated to a single item has been reduced as the number of analysis items increases. In some cases, the sample itself is valuable, and therefore, cannot be prepared in large amounts. Accordingly, an analysis of microscale samples which has heretofore been considered a high-level analysis has become a routine practice. In addition, reagents generally tend to be costly as the details of analysis become increasingly sophisticated. Thus, there is a demand for a reduction in the amounts of reagents also from the viewpoint of costs. Such reductions in the amounts of samples and reagents also serve as a strong incentive to promote the miniaturization of reaction cells. In addition, the miniaturization of reaction cells and reductions in the amounts of required samples and reagents, offer the advantages of leading to an improvement in analysis throughputs and a reduction in the amount of waste liquid. In that case, there is the need to precisely pipette microscale samples and reagents.
Here, a mechanism for automatically injecting samples and reagents in a commonly used autoanalyzer is referred to as a pipetting nozzle or a pipetting probe, and is generally made of metal, glass or resin. In addition, a reaction cell (also referred to as a cell, a reaction container or a well) used in the commonly used autoanalyzer is generally formed of glass, synthetic resin (plastic), or the like. According to Patent Literature 2, a suction/discharge method is available as one example of pipetting methods. By suctioning and discharging a sample by using a sample pipetting nozzle, the sample can be transferred, for example, from a container, such as a blood sampling tube, in which the sample is stored, to a reaction cell for reacting the sample with a reagent.
Examples of the sample pipetting nozzle include a reusable metal pipetting nozzle and a disposable plastic pipetting nozzle. The disposable nozzle is convenient in that the nozzle need not be cleaned at each time of use, but is not economical. The reusable nozzle is economical but needs to be cleaned at each time of reuse. Hence, insufficient cleaning results in that a nozzle to which residues of the sample are attached is used the next time. In this case, there arises the problem of so-called carry-over in which residues of the sample get mixed in with a newly collected sample.
As another example of pipetting methods, an ink-jet method shown in Patent Literature 3 is available. A trend in the field of autoanalyzers is toward further reductions in the amounts of samples and reagents. In addition, a demand for the miniaturization of apparatus continues to grow further. Samples are blood serum and urine and the minimum pipetted amount of each of these liquid samples has fallen below 2 μL in recent years. In addition, the diameter of nozzles has been reduced to approximately 0.5 mm. As the diameter is reduced in this way, a ratio of the contact area to the volume of a nozzle increases. Thus, there is the need to control substances adsorbed onto the surfaces of the nozzle. In this case, the nozzle is used after cleaning off substances adsorbed onto the inner and outer surfaces of the nozzle with a cleaning liquid using water or surfactant, when a plurality of samples is successively measured. This cleaning may be carried out at each time of pipetting or at least at the time of transfer from one sample to another. As cleaning methods, water or a cleaning liquid is soused for the outer surfaces of the nozzle or water is pushed out from the back end of a flow path within the nozzle for the inner surfaces thereof. As another cleaning method, the nozzle is cleaned by performing suction and discharge by using a cleaning liquid containing a surfactant at the end of a certain period of time, for example, at the end of a day's work.